Protein tyrosyl phosphorylation plays a central role in the regulation of cell growth and differentiation. The levels of protein tyrosyl phosphorylation are controlled by the dual action of protein-tyrosyl kinases (PTKs) and protein-tyrosyl phosphatases (PTPs). Not much is known about the function of PTPs or their role in disease. The aim of this proposal is to investigate the role of the SH2-containing PTP, SHPTP1, in signaling from the erythropoietin receptor (EPOR). Mutation of SHPTP1 in the motheaten mouse leads to a panoply of hematopoietic abnormalities including EPO hypersensitivity. In cultured cells, SHPTP1 appears to act as a negative regulator of the EPOR pathway by dephosphorylating and inactivating the EPOR-bound PTK, JAK2. The specific aim of the research will be to study the detailed molecular mechanism of the EPOR/SHPTP1 interaction and its effect on JAK2. Site directed mutagenesis of the receptor will be used to unambiguously locate the site of SHPTP1 binding. Additional mutagenesis will help us determine if the EPOR/SHPTP1 interaction is necessary and sufficient for JAK2 inactivation. The lag between the recruitment of the PTP to the receptor and the onset of JAK2 inactivation will be addressed by studying post-translational modifications of the two enzymes. Finally the significance of this interaction in the whole animal will be explored using transgenic mice.